1. Field of the Invention
This invention generally relates to electromagnetic radiation shielding for electrical connectors and, more particularly, to a shield for controlling radiation associated with an electrical connector having a non-continuous counterpoise.
2. Description of the Related Art
As noted in U.S. Pat. No. 6,849,800, Mazurkiewicz, electromagnetic emissions are the unwanted byproduct of high-frequency electronic signals necessary, for example, to operate an electronic microprocessor, logic circuitry, or a radio frequency (RF) antenna. The resulting electromagnetic interference (EMI) is problematic when it interferes with licensed communications such as cellular telephones, nearby electrical circuits, and connected electrical equipment. This type of interference may also be known as radio-frequency interference (RFI).
To meet EMI regulations or otherwise control radiated emissions, electronic equipment may employ a combination of two approaches commonly referred to as “source suppression” and “containment.” Source suppression attempts to design components and subsystems such that only essential signals are present at signal interconnections, and that all non-essential radio frequency (RF) energy is either not generated or attenuated before it leaves the component subsystem. Containment attempts conventionally include placing a barrier around the assembled components, subsystems, and interconnections, to retain unwanted electromagnetic energy within the boundaries of the product where it is harmlessly dissipated.
This latter approach, containment, is based on a principle first identified by Michael Faraday (1791-1867), that a perfectly conducting box completely enclosing a source of electromagnetic emissions prevents those emissions from leaving the boundaries of the box. This principle is employed in shielded cables as well as in conventional shielded enclosures. Conventional shielded enclosures are typically implemented as a metal box or cabinet that encloses the equipment. The metal box is commonly referred to as a metallic cage and is often supplemented with additional features in an attempt to prevent RF energy from escaping via the power cord and other interconnecting cables. For example, a product enclosure might consist of a plastic structure with a conductive coating on the surface. This approach is commonly implemented in, for example, cell phones. More commonly, the metal enclosure is implemented as a metal cage located inside the product enclosure. Since the EMI suppression necessary for the entire product or system requires that only a portion of the product be shielded, such metallic cages are commonly placed around selected components or subsystems.
There are numerous drawbacks to the use of such metallic cages primarily relating to the lack of shielding effectiveness. Electromagnetic energy often escapes the metallic cage at gaps between the metallic cage and the printed circuit board. Electrical gaskets and spring clips have been developed to minimize such leakage. Unfortunately, such approaches have only limited success at shielding while increasing the cost and complexity of the printed circuit board. In addition, leakage occurs because the cables and wires penetrating the metallic cage are not properly bonded or filtered as they exit the metallic cage. Further drawbacks of metallic cages include the added cost and weight to the printed circuit board assembly, as well as the limitations placed on the package design.
High frequency signals are communicated via cables, wiring, or across circuit boards based upon the principle that the signal-carrying medium can be formed into a (LC) transmission line. To that end, coaxial cables are formed from a center signal conductor and an outer coaxial ground. Signals can also be carried via a twisted-pair of wires. Microstrip circuit boards are made with a signal trace, coplanar grounds, and an underlying groundplane. However, when changing from one medium to another, a large voltage standing wave ratio (VSWR) may be created at the interface. For example, the interface between a coax cable and a microstrip circuit board may be a board mounted SMA connector that brings the signals off the board using vertical pins. At this interface, the ideal transmission line characteristics may be flawed, and the high VSWR may cause the conducted signal to radiate. Also, the contacts between push-on or screw/threaded coaxial connectors may have a high VSWR, resulting in unintentional radiation or other susceptibility to other radiation sources.
A conventional USB cable, such as might be used to connect a personal computer (PC) with a printer, provides another example of an unintended radiation problem. The ground signal from the computer is generally carried in the cable shield surrounding the signal wire. However, the cable/PC interface is a push-on connector that is likely to “leak” radiation. One common attempt to address this problem is the use of a ferrite bead or core. For example, a PC power cable may pass through one or more ferrite cores. The core mitigates against conducted radiation on the outside of the cable, but it does not address the problem at its source.
Other types of connections include a non-continuous counterpoise by necessity. For example, there may be no explicit ground (counterpoise) connection when a monopole antenna is connected to a coax cable or a microstrip board, as the radiated antenna energy may be designed to return to ground via other paths. Even for antennas having a counterpoise, a poor interface can become an unintended radiator. Alternately, a non-continuous counterpoise antenna connection becomes a likely entry place for unintended radiators and component noise that couple into a received RF signal, compromising receiver sensitivity.
The energy radiated from connector interfaces can be detrimental to proximate electrical circuits. In a wireless telephone for example, the energy radiated from an antenna connection can create “hotspots” on a telephone circuit board. A hotspot near a sensitive RF receiver may result in autojamming. Likewise, the jamming effect can result from energy being coupled into the circuit board from a cable-connected accessory. Alternately, a hotspot may result in component noise coupling with a signal that is transmitted by the antenna.
A large number of connectors designs exist based upon the above-mentioned metallic cage approach. The effectiveness of the designs is usually balanced against practical considerations such as size, complexity, cost, assembly time, and durability. Less attenuation has been paid to the shielding of antenna connectors, as the focus is usually centered on the ability of the antenna to effectively radiate. Some solutions involve shielding sensitive electrical circuits, as opposed to stopping the radiation at its source.
It would be advantageous if a simple, low-cost shield existed that effectively contained electromagnetic energy radiating from an electrical connector.